PROJECT DESCRIPTION: This SBIR Phase II proposal is intended to further develop recombinant human MFG-E8 (rhMFG-E8) as a radiation medical countermeasure (MCM) towards its approval by the FDA in the future. Nuclear terrorism and major nuclear power plant leaks can cause acute radiation injury on a large scale. Currently, there are limited drugs available to treat acute radiation syndrome (ARS). rhMFG-E8 is a secretory glycoprotein that can maintain intestinal barrier homeostasis, enhance the clearance of dying cells, and reduce inflammation. In our Phase I studies, we have shown that endogenous MFG-E8 is downregulated after radiation injury, and treatment with E. coli-expressed His-tagged rhMFG-E8 improved the survival, body weight, and intestinal integrity of rats exposed to gamma irradiation. However, E. coli-expressed His-tagged proteins are inappropriate for the use in humans. Therefore, we have generated tag-free rhMFG-E8 using human cells, significantly enhancing rhMFG-E8?s biological activities. We have shown that tag-free rhMFG-E8 significantly improved the survival, body weight, and intestinal integrity of mice exposed to X-ray irradiation. We also determined rhMFG-E8?s pharmacokinetics, possible lack of mutagenicity, and short-term (3 months) stability. Based on the above positive results, we hypothesize that human cell-expressed tag-free rhMFG-E8 can be developed as an effective and safe post-exposure mitigator of acute radiation injury. In this proposal, we will assess tag-free rhMFG-E8?s efficacy as a radiation MCM in mice with the gastrointestinal acute radiation syndrome (GI-ARS) and hematopoietic acute radiation syndrome (H-ARS). In addition, we will determine the therapeutic window and effects of reduced treatment duration after radiation exposure, as well as rhMFG-E8?s safety and potential oncogenicity and immunogenicity. These proposed studies should provide crucial information on the efficacy and safety of rhMFG-E8 as a novel radiation MCM targeting GI-ARS and combined GI- and H-ARS. Our ultimate goal is to obtain FDA approval to use rhMFG-E8 as a safe and effective treatment for victims suffering from severe ARS.